The article discusses research entitled "Regulation of intestinal inflammation by microbiota following allogeneic bone marrow transplantation," by R. R. Jeng and colleagues. In this study, the authors show how gut commensals influence the severity of graft-versus-host disease (GVHD) following allogeneic bone marrow transplantation (BMT). It suggests that antibiotic exposure prior to BMT could be a risk factor for GVHD.